Calibration of digital cameras and other imaging devices seeks to create a mathematical model of how the image ‘prints’ through the lens on the imaging device's surface. The procedure first uses a picture from a calibration target with accurately known tolerance, and extracts target elements from the image. Finally, a mathematical model relates the image information with the real three-dimensional (3D) target information. Once calibrated, the imaging device can then be used to map real world objects using a scale factor, the focal distance f. When working from off-the-shelf cameras and lenses, we need to calibrate the camera to compensate for the tolerance on the lens focal distance, where the tolerance can be as high as 10%.
Moreover, once the model is accurately known, it can then be used to recreate a perfect camera image, also referred to as pinhole, needed for almost every high end automated imaging system. Through software image correction, we can compensate for image errors introduced by the imperfect nature of lenses, fish eye image deformation called geometric distortion, and rainbow light splitting in the lens optics called chromatic distortion. Several imaging devices will exhibit an off-squareness line of sight bias with respect to the image plane. In order to properly measure image distortion, the off-squareness of the image plane with respect to the lens line of sight needs to be compensated for. Known calibration techniques use the tilted axis assumption for this purpose. However, this assumption has proven to bias every camera parameter in the model, causing systematic measurement errors. As a result, a scale-size-distortion bias is introduced in the image that every other camera parameter seeks to compensate, biasing those camera parameters as well. In 3D scanning or telemetry, this translates into a geometry and location bias of 3D objects when reconstructing from a pair of simultaneous camera images.
There is therefore a need to improve on existing calibration and modeling techniques for imaging devices.